This invention relates to a tapping machining method for moving a tapping tool to a commanded position in synchronism with rotation of a spindle to subject a workpiece to threading, thereafter rotating the spindle in the reverse direction and withdrawing the tapping tool from the workpiece in synchronism with rotation of the spindle.
In tapping machining, a tapping tool mounted on a spindle is moved relative to a workpiece in the depth direction in synchronism with rotation of the spindle to subject the workpiece to a predetermined threading operation. The spindle is subsequently rotated in the reverse direction and the tapping tool is moved relative to the workpiece in the withdrawing direction in synchronism with rotation of the spindle, to withdraw the tapping tool from the workpiece.
FIG. 1 is a cross-sectional view for describing such a tapping machining method. A workpiece 3 in which a bore has been formed in advance is placed upon a table 1. A helical cutting edge 5 is formed on the outer periphery of a tapping tool 4 mounted on a spindle (not shown). A female screw 6 is formed in the workpiece 2 when the tapping tool is moved relative to the workpiece 2 along the bore 2 in synchronism with the rotation of the spindle.
In such tapping machining, the sequence followed is to move the tapping tool to a commanded position in the depth direction (along the Z axis) in synchronism with rotation of the spindle, halt the movement of the tapping tool in the depth direction after the commanded position is reached, then reverse the rotation of the spindle and move the tapping tool in a direction which will withdraw it from the workpiece. Though a spindle reverse rotation command for reversing the rotation of the spindle is issued after the commanded position is reached, the spindle does not reverse instantaneously but continues rotating forwardly for a time due to inertia. The screw threads will be flattened and a highly precise tapping machining operation will be impossible unless the tapping tool is moved in the depth direction during the inertial rotation of the spindle. Accordingly, the conventional practice is to use an elongatable tool, referred to as a tapper, as the tapping tool. The tapper is adapted to elongate during interial rotation and contract during withdrawal, and is capable of moving in the depth direction even during inertial rotation.
However, the tapper is costly since it is specially contrived. To cut a screw having a large diameter, moreover, the tapper itself must be large in size, thus resulting in structural problems. In addition, the bottom of the bore cannot be cut to a high precision when the tapper is used. Accordingly, there is a need for a method whereby tapping can be carried out with good precision using a rigid tapping tool that is not elongatable. Heretofore, however, the aforementioned problem of screw thread flattening caused by inertial rotation has not been solved.